The present invention relates in general to electric motor controls, and in particular to a new and useful apparatus and method of controlling the speed of an electric motor in a manner which avoids synchronous speed variations.
U.S. Pat. No. 4,527,101 discloses an apparatus for measuring the speed of an electric motor which performs a fast Fourier transform analysis on variations in current being supplied to an electric motor. Current variations are measured as voltage changes over a resistor and the signal is filtered to remove frequencies which are not related to the motor speed, such as the power supply frequency, and to amplify the signal before it is supplied to an A/D to be processed as a digital signal within a microprocessor. The microprocessor utilizes a synchronizing pulse generated by the motor and supplied by a sync detector to the microprocessor. A fast Fourier transform is performed on the digital values and produces an amplitude spectrum with the maximum spectral component being taken as the speed of the motor. This speed measurement is then utilized by a controller to feed a gate signal CTRL back to a Thyristor that controls the power supply to the motor. The synchronization signal of this reference is the zero crossing of the AC power waveform. Actual variations in motor shaft rotation may exist which are not reflected in the zero crossing. There is also no teaching of synchronous time averaging (STA) to remove the random fluctuations from the signal, and the subsequent generation of an antiwave which is applied to the controller for compensating for the speed fluctuations as in the present invention.
U.S. Pat. No. 4,739,230 discloses an anti-hunting motor controller which determines the rotational speed of a motor based on the rotational position of its shaft or equivalent, in a rotational sensor. A phase comparitor is then utilized to determine whether the speed must be increased or decreased. The controller includes a plurality of different patterns which can be selected for controlling the motor speed in the most advantageous manner.
U.S. Pat. No. 3,896,356 addresses the problem of damping oscillations in the output speed of a motor. Either the voltage or synchronous frequency being applied to the motor is responsive to motor performance. PID techniques are utilized in the feedback loop.
None of these references uses fast Fourier analysis and synchronous time averaging to predict future periodic perturbations and construct an antiwave which is applied to the motor controller, as in the present invention.
Synchronous time averaging has been used to measure vibrations in the rolls of a paper machine to detect undesirable vibrations in the rolls which degrade the paper quality. See "Low-Frequency Vibration Analysis of Paper Machine Presses Using Displacement Transducers and the Synchronous Time-Averaging Method", Y. P. Gagnom, Jul. 1987, Tappi Journal. Information obtained using synchronous time averaging was utilized in this reference to locate defective rolls for replacement.
Fast Fourier transform analysis (FFTA) has been used to analyze vibrations in a variety of fields, including to identify defects in gears in a gearbox. See, for example "Vibration Analysis Pinpoints Gear Defects", E. A. Page, et al., Power Transmission Design, March 1989. Despite the existence of a variety of analysis and control techniques, including STA and FFTA, major problems remain in achieving the accurate constant speed control of electric motors. This includes the use of built in error in systems which asynchronously wait for actual errors to occur and require a hunting period before the error is compensated.
Noise compensation technology is also known which seeks to reduce noise having periodic characteristics such as the noise of fans, machinery, helicopter rotors and the like. For this purpose, an out of phase antiwave is generated and played through amplifiers and speakers.